Methylenecyclopropane Rearrangement as a Probe for Free Radical Substituent Effects. ?. Values for Commonly Encountered Conjugating and Organometallic Groups

Article abstract of DOI:10.1021/jo00391a015

A series of 3-aryl-2,2-dimethylmethylenecyclopropanes, 8, with NO2, NMe2, vinyl, isopropenyl, phenyl, cyclopropyl, CH2SiMe3, SiMe3, SnMe3, , and HgCl substitution in the para position of the aromatic ring have been prepared.All rearrange thermally to the corresponding isopropylidenecyclopropanes 9 at rates that are substituent dependent.These commonly encountered substituents all enhance rearrangement rates relative to the unsubstituted analogue with p-NMe2 being the most effective.The rate enhancements are interpreted in terms of stabilization of the biradical intermediate by the para substituent.Rate data have allowed the assignment of ?^. values for these groups, which have not been previously determined.The nitro group in the para position is also quite effective in increasing the rearrangement rate, which contrasts with the effect of this group on many other free radical reactions.Vinyl is somewhat more effective as a radical stabilizing group than is isopropenyl or phenyl, possibly due to steric interactions in the planar conformations necessary for conjugative stabilization by isopropenyl or phenyl.Trimethylsilyl,trimethylstannyl, and HgCl all enhance the methylenecyclopropane rearrangement rate, but only to a moderate extent.Boron containing substituents, where boron can act as an acceptor group, are among the more effective radical stabilizing groups, as implied by their effect on the rearrangement rate of 8.The cyclopropyl and CH2SiMe3 groups, which also enhance the rearrangement rate of 8 to a moderate extent, become even more effective radical stabilizing groups when present in conjunction with the carbethoxy group.These two conjugating groups are therefore capable of acting as donor groups in captodative radical stabilization.